Vein filters are used for the prevention of pulmonary embolism. A collapsible filter is introduced into the vein through a catheter, usually in the inferior vena cava, to prevent blood clots from travelling to the pulmonary artery. Removable filters are designed to be used temporarily, for example, during surgery, and then be removed. They can also be left permanently in the patient if the patient's condition requires permanent protection. Convertible filters are designed for temporary use and can be transformed into a stent when the filter function is no longer required.
The filters are collapsible so that they can be introduced in their collapsed state into the inner lumen of a catheter and placed in a vein by release from the catheter. The distal end of the catheter is positioned at the desired location of the filter insertion and the latter is then released from the catheter. For this purpose, the filter is usually held in its position relative to the vein by a support device on its proximal end and then the catheter is removed gradually. The collapsed filter expands out of the distal end of the catheter and is deployed into the vein.
A common type of filter is composed of metallic legs that open in an umbrella like manner, grouped on the proximal end at a filter head. In the collapsed state inside the catheter, the legs are under tension and are oriented approximately parallel to each other. Outside of the catheter, because of their elasticity, the legs move apart so that the filter can be deployed into the vein. At least some legs have on their ends barbed hooks to embed the filter in the walls of the blood vessel.
To be convertible, a filter must keep a closed structure after removal of the filter head to transform the filter into a stent configuration. This requires additional connecting wires which connect the filter legs in the stent configuration. The document WO 2006/107939 A1 describes such a filter.
The application of convertible vein filters is however limited by the diameter of the collapsed filter to be introduced into the catheter lumen for insertion of the filter into a vessel. The diameter of the filter head, where all filter legs meet, limits the miniaturization of such filters.